In recent years, a rapid progress has been achieved in the field of electronics. One reason therefor exists in that extremely highly integrated semiconductor devices have been put into practice. A dry etching technique is very important for forming a fine pattern on a silicon wafer for achieving the high integration, and is constantly progressing.
In dry etching, in order to produce fluorine-containing active ingredients by plasma discharge or other means, gases of compounds containing many fluorine atoms have heretofore been used as the etching gas. As examples of the fluorine-containing etching gas, there can be mentioned highly fluorinated compounds such as carbon tetrafluoride, sulfur hexafluoride, nitrogen trifluoride, carbon trifluoro-monobromide, trifluoromethane, hexafluoroethane and octafluoropropane.
International efforts for conserving the global environment are being made, and especially those for preventing or minimizing the global warming are now attracting a great interest. For example, in IPCC (Intergovernmental Panel on Climate Change), regulations on control of the total amount of carbon dioxide emission have been established in the international agreement. Under these circumstances, it is pointed that, from a viewpoint of prevention of the global warming, there is an increasing need of developing alternatives for the highly fluorinated compounds which have heretofore widely been used and which have a long life in the air and cause the global warming. More specifically, it is said that carbon tetrafluoride, hexafluoroethane and sulfur hexafluoride have a life in the air of 50,000, 10,000 and 3,200 years, respectively. These fluorinated compounds exhibit a large absorbability for infrared rays and exert a considerable influence upon the global warming. Thus, it is eagerly desired to develop an etching process utilizing a novel etching gas which does not cause the global warming and has etching performances comparable to those of the heretofore used etching gases.
Various proposals have been made for enhancing the selectivity to a protective thin film such as a photoresist or a polysilicon in a dry etching process. For example, a proposal has been made in Japanese Unexamined Patent Publication No. H4-170026 wherein a silicon compound is etched by using a gas containing an unsaturated fluorocarbon such as perfluoropropene or perfluorobutene while the temperature of the substrate to be etched is controlled to a temperature not higher than 50.degree. C. Another proposal has been made in Japanese Unexamined Patent Publication no. H4-258117 wherein etching is effected in a manner similar to the above proposal by using a gas containing a cyclic saturated or cyclic unsaturated fluorocarbon such as perfluorocyclopropane, perfluorocyclobutane, perfluorocyclobutene or perfluorocyclopentene, while the temperature of the substrate to be etched is controlled to a temperature not higher than 50.degree. C.
However, the dry etching techniques heretofore proposed in the above patent publications have problems such that the etching gas is not sufficiently decomposed by the plasma discharge, depending upon the particular dry etching conditions, to polymerize on the substrate whereby a polymer deposition in the form of a brown thin film is produced on the substrate, and further that the rate of etching is not high and the selectivity to a protective thin film such as a photoresist or a polysilicon is low.